Question: Simplify the following expression: $a = \dfrac{3z}{10} - \dfrac{5z}{7}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $10$ and $7$ $\lcm(10, 7) = 70$ $ a = \dfrac{7}{7} \cdot \dfrac{3z}{10} - \dfrac{10}{10} \cdot \dfrac{5z}{7} $ $a = \dfrac{21z}{70} - \dfrac{50z}{70}$ $a = \dfrac{21z -50z}{70}$ $a = \dfrac{-29z}{70}$